The thermite reaction has been utilized for a long period of time in order to provide weldments normally associated with the railroad industry. The thermite reaction is comprised of a reaction between aluminum and a metal oxide wherein the metal oxide is reduced providing a filler metal, i.e., the source of the filler metal is the oxide on reduction. The "Goldschmidt" reaction is the basis of the application of the process described in U.S. Pat. No. 2,229,045 to Charles A. Cadwell. This reaction is as follows: ##STR1##
The "Goldschmidt" reaction has been successfully utilized over the years to weld or join metals such as iron (Fe) and copper (Cu). However, the process is not particularly well adapted for use in joining metals such as aluminum (Al) wherein the use of a copper (Cu) or iron (Fe) filler metal is generally unacceptable.
U.S. Pat. No. 3,020,610 to Rejdak discloses a method of welding aluminum (Al) and other metals, and provides a listing of various reactions which can be utilized to provide a weldment. Unfortunately, in the method disclosed by Rejdak the weld which is formed is not suitable for some applications. For example, a weldment produced by the Rejdak method in some environments (such as continuous immersion in rainwater, for example) may be susceptible to a high level of corrosion due to the presence of impurities or unwanted elements in the resultant weldment.
U.S. Pat No. 2,569,956 to Schiltknecht discloses a process wherein a thermite reaction is utilized to join together a pair of nonferrous metal pieces by melting a sleeve about the metal pieces. This process is usually referred to as the "Alusuisse Process" referring to a welding system produced by Swiss Aluminum Ltd. of Zurich.
U.S. Pat No. 5,062,903 to Brosnan discloses a process whereby a reducing agent is used to produce heat to melt a filler metal, which then flows out to provide a weldment. Other product components are left behind in a slag with reaction gases released. The process is generally summarized as follows: EQU Reducing agent+filler metal+metallic compound=heat+weld metal+slag+gases
The Brosnan invention was used to weld aluminum metal by using aluminum powder as both the reducing agent and filler metal, and calcium sulfate (CaSO.sub.4) as the metallic compound. The basic reaction is: EQU Al+CaSO.sub.4 =heat+Al.sub.2 O.sub.3 +CaS+Al
The weld metal produced by this reaction was essentially pure (99%) aluminum (Al). However, the weldments had much lower strengths than that of the aluminum pieces to be welded. The solidified weld metal also had high levels of porosity in some instances.
U.S. Pat. No. 5,171,378 to Kovarik discloses an invention which improves on the Brosnan process through the addition of silicon, (Si), which strengthens the weld metal considerably. The porosity is also reduced through the addition of sodium chloride (NACl).
One major drawback of both the Brosnan and Kovarik processes for welding aluminum is that the weld and slag give off a strong unpleasant odor of hydrogen sulfide (H.sub.2 S) after the weld has been made. This severely limits commercial acceptance of such an invention.